In Situ Etch Rate Measurements by Alpha-Particle Energy Loss

ABSTRACTWhen alpha-particles pass through thin films they lose an amount of energy proportional to the film thickness with the proportionality constant depending on the film composition. Thus, by measuring this energy loss one can determine the film thickness. We have applied this technique to measurements of the etch rate of various III-V semiconductor layers grown by molecular beam epitaxy. Prior to film growth, GaAs substrates were recoil-implanted with the alpha-emitting 224Ra isotope by exposure to a 5µCi source of 228Th. The implanted isotope decays with a half-life of 3.7 days, which allows measurements to be done for up to about two weeks after implantation. Following growth, the samples were etched in an electron cyclotron resonance etcher using a Cl2/BCl3/Ar gas mixture. As the film is etched the energy of the alpha-particles emitted from the surface increases. By introducing a high resolution Si detector into the etcher we are able to measure changes in the alpha-emission spectrum without removing the sample from the etcher. Thickness changes with an uncertainty of 5–10nm are obtained in 5 minute measurements at the end of each etch step. Some of the samples were also measured by SEM, yielding results in good agreement with values obtained by the alpha-particle measurements. As an example of an application of the technique we will describe measurements of the temperature dependence of the etch rate of GaAs in the 15–150 °C temperature range using optical bandgap thermometry to determine the substrate temperature. In a second example, we explore the application of the technique to etch rate of short pitch (250–500nm) grating. In this case the shape of the alpha-spectrum is sensitive to the profile of the etched trenches.